rails--rails/guides/source/active_model_basics.md

Active Model Basics
===================

This guide should provide you with all you need to get started using model
classes. Active Model allows for Action Pack helpers to interact with
plain Ruby object. Active Model also helps building custom ORMs for use
outside of the Rails framework.

After reading this guide, you will be able to add to plain Ruby objects:

* The ability to behaves like an Active Record model.
* Add callbacks and validations like Active Record.
* Add serializers.
* Integrate with the Rails internationalization (i18n) framework.

--------------------------------------------------------------------------------

Introduction
------------

Active Model is a library containing various modules used in developing
classes that need some features present on Active Record.
Some of these modules are explained below.

### ActiveModel::AttributeMethods

The `ActiveModel::AttributeMethods` module can add custom prefixes and suffixes
on methods of a class. It is used by defining the prefixes and suffixes and
which methods on the object will use them.

```ruby
class Person
  include ActiveModel::AttributeMethods

  attribute_method_prefix 'reset_'
  attribute_method_suffix '_highest?'
  define_attribute_methods 'age'

  attr_accessor :age

  private
    def reset_attribute(attribute)
      send("#{attribute}=", 0)
    end

    def attribute_highest?(attribute)
      send(attribute) > 100
    end
end

person = Person.new
person.age = 110
person.age_highest?  # true
person.reset_age     # 0
person.age_highest?  # false
```

### ActiveModel::Callbacks

`ActiveModel::Callbacks` gives Active Record style callbacks. This provides an
ability to define callbacks which run at appropriate times.
After defining callbacks, you can wrap them with before, after and around
custom methods.

```ruby
class Person
  extend ActiveModel::Callbacks

  define_model_callbacks :update

  before_update :reset_me

  def update
    run_callbacks(:update) do
      # This method is called when update is called on an object.
    end
  end

  def reset_me
    # This method is called when update is called on an object as a before_update callback is defined.
  end
end
```

### ActiveModel::Conversion

If a class defines `persisted?` and `id` methods, then you can include the
`ActiveModel::Conversion` module in that class and call the Rails conversion
methods on objects of that class.

```ruby
class Person
  include ActiveModel::Conversion

  def persisted?
    false
  end

  def id
    nil
  end
end

person = Person.new
person.to_model == person  # => true
person.to_key              # => nil
person.to_param            # => nil
```

### ActiveModel::Dirty

An object becomes dirty when it has gone through one or more changes to its
attributes and has not been saved. This gives the ability to check whether an
object has been changed or not. It also has attribute based accessor methods.
Let's consider a Person class with attributes `first_name` and `last_name`:

```ruby
require 'active_model'

class Person
  include ActiveModel::Dirty
  define_attribute_methods :first_name, :last_name

  def first_name
    @first_name
  end

  def first_name=(value)
    first_name_will_change!
    @first_name = value
  end

  def last_name
    @last_name
  end

  def last_name=(value)
    last_name_will_change!
    @last_name = value
  end

  def save
    # do save work...
    changes_applied
  end
end
```

#### Querying object directly for its list of all changed attributes.

```ruby
person = Person.new
person.changed? # => false

person.first_name = "First Name"
person.first_name # => "First Name"

# returns if any attribute has changed.
person.changed? # => true

# returns a list of attributes that have changed before saving.
person.changed # => ["first_name"]

# returns a hash of the attributes that have changed with their original values.
person.changed_attributes # => {"first_name"=>nil}

# returns a hash of changes, with the attribute names as the keys, and the values will be an array of the old and new value for that field.
person.changes # => {"first_name"=>[nil, "First Name"]}
```

#### Attribute based accessor methods

Track whether the particular attribute has been changed or not.

```ruby
# attr_name_changed?
person.first_name # => "First Name"
person.first_name_changed? # => true
```

Track what was the previous value of the attribute.

```ruby
# attr_name_was accessor
person.first_name_was # => "First Name"
```

Track both previous and current value of the changed attribute. Returns an array
if changed, else returns nil.

```ruby
# attr_name_change
person.first_name_change # => [nil, "First Name"]
person.last_name_change # => nil
```

### ActiveModel::Validations

`ActiveModel::Validations` module adds the ability to class objects to validate
them in Active Record style.

```ruby
class Person
  include ActiveModel::Validations

  attr_accessor :name, :email, :token

  validates :name, presence: true
  validates_format_of :email, with: /\A([^\s]+)((?:[-a-z0-9]\.)[a-z]{2,})\z/i
  validates! :token, presence: true
end

person = Person.new(token: "2b1f325")
person.valid?                        # => false
person.name = 'vishnu'
person.email = 'me'
person.valid?                        # => false
person.email = 'me@vishnuatrai.com'
person.valid?                        # => true
person.token = nil
person.valid?                        # => raises ActiveModel::StrictValidationFailed
```

### ActiveModel::Naming

Naming adds a number of class methods which make the naming and routing
easier to manage. The module defines the `model_name` class method which
will define a number of accessors using some `ActiveSupport::Inflector` methods.

```ruby
class Person
  extend ActiveModel::Naming
end

Person.model_name.name                # => "Person"
Person.model_name.singular            # => "person"
Person.model_name.plural              # => "people"
Person.model_name.element             # => "person"
Person.model_name.human               # => "Person"
Person.model_name.collection          # => "people"
Person.model_name.param_key           # => "person"
Person.model_name.i18n_key            # => :person
Person.model_name.route_key           # => "people"
Person.model_name.singular_route_key  # => "person"

### ActiveModel::Model

`ActiveModel::Model` adds the ability to a class to work with Action Pack and
Action View out of box.

```ruby
class EmailContact
  include ActiveModel::Model

  attr_acessor :name, :email, :message
  validates :name, :email, :message, presence: true

  def deliver
    if valid?
      # deliver email
    end
  end
end
```

When including `ActiveModel::Model` you get some features like:

- model name instrospection
- conversions
- translations
- validations

It also gives you the ability to initialize an object with a hash of attributes,
much like any Active Record object.

```ruby
email_contact = EmailContact.new(name: 'David',
                                 email: 'david@example.com',
                                 message: 'Hello World')
email_contact.name       # => 'David'
email_contact.email      # => 'david@example.com'
email_contact.valid?     # => true
email_contact.persisted? # => false
```

Any class that includes `ActiveModel::Model` can be used with `form_for`,
`render` and any other Action View helper methods, just like Active Record
objects.

### ActiveModel::Serialization

`ActiveModel::Serialization` provides a basic serialization for your object.
You need to declare an attributes hash which contains the attributes you want to
serialize. Attributes must be strings, not symbols.

```ruby
class Person
  include ActiveModel::Serialization

  attr_accessor :name

  def attributes
    {'name' => nil}
  end
end
```

Now you can access a serialized hash of your object using the `serializable_hash`.

```ruby
person = Person.new
person.serializable_hash   # => {"name"=>nil}
person.name = "Bob"
person.serializable_hash   # => {"name"=>"Bob"}
```

#### ActiveModel::Serializers

Rails provides two serializers `ActiveModel::Serializers::JSON` and
`ActiveModel::Serializers::Xml`. Both of these modules automatically include
the `ActiveModel::Serialization`.

##### ActiveModel::Serializers::JSON

To use the `ActiveModel::Serializers::JSON` you only need to change from
`ActiveModel::Serialization` to `ActiveModel::Serializers::JSON`.

```ruby
class Person
  include ActiveModel::Serializers::JSON

  attr_accessor :name

  def attributes
    {'name' => nil}
  end
end
```

With the `as_json` you have a hash representing the model.

```ruby
person = Person.new
person.as_json # => {"name"=>nil}
person.name = "Bob"
person.as_json # => {"name"=>"Bob"}
```

From a JSON string you define the attributes of the model.
You need to have the `attributes=` method defined on your class:

```ruby
class Person
  include ActiveModel::Serializers::JSON

  attr_accessor :name

  def attributes=(hash)
    hash.each do |key, value|
      send("#{key}=", value)
    end
  end

  def attributes
    {'name' => nil}
  end
end
```

Now it is possible to create an instance of person using the `from_json`.

```ruby
json = { name: 'Bob' }.to_json
person = Person.new
person.from_json(json) # => #<Person:0x00000100c773f0 @name="Bob">
person.name            # => "Bob"
```

##### ActiveModel::Serializers::Xml

To use the `ActiveModel::Serializers::Xml` you only need to change from
`ActiveModel::Serialization` to `ActiveModel::Serializers::Xml`.

```ruby
class Person
  include ActiveModel::Serializers::Xml

  attr_accessor :name

  def attributes
    {'name' => nil}
  end
end
```

With the `to_xml` you have a XML representing the model.

```ruby
person = Person.new
person.to_xml # => "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<person>\n  <name nil=\"true\"/>\n</person>\n"
person.name = "Bob"
person.to_xml # => "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<person>\n  <name>Bob</name>\n</person>\n"
```

From a XML string you define the attributes of the model.
You need to have the `attributes=` method defined on your class:

```ruby
class Person
  include ActiveModel::Serializers::Xml

  attr_accessor :name

  def attributes=(hash)
    hash.each do |key, value|
      send("#{key}=", value)
    end
  end

  def attributes
    {'name' => nil}
  end
end
```

Now it is possible to create an instance of person using the `from_xml`.

```ruby
xml = { name: 'Bob' }.to_xml
person = Person.new
person.from_xml(xml) # => #<Person:0x00000100c773f0 @name="Bob">
person.name          # => "Bob"
```

### ActiveModel::Translation

Provides integration between your object and the Rails internationalization
(i18n) framework.

```ruby
class Person
  extend ActiveModel::Translation
end
```

With the `human_attribute_name` you can transform attribute names into a more
human format. The human format is defined in your locale file.

* config/locales/app.pt-BR.yml

  ```yml
  pt-BR:
    activemodel:
      attributes:
        person:
          name: 'Nome'
  ```

```ruby
Person.human_attribute_name('name') # => "Nome"
```

### ActiveModel::Lint::Tests

Test whether an object is compliant with the Active Model API.

* app/models/person.rb

    ```ruby
    class person
      include ActiveModel::Model

    end
    ```

* test/models/person_test.rb

    ```ruby
    require 'test_helper'

    class PersonTest < ActiveSupport::TestCase
      include ActiveModel::Lint::Tests

      def setup
        @model = Person.new
      end
    end
    ```

```bash
$ rake test

Run options: --seed 14596

# Running:

......

Finished in 0.024899s, 240.9735 runs/s, 1204.8677 assertions/s.

6 runs, 30 assertions, 0 failures, 0 errors, 0 skips
```

An object is not required to implement all APIs in order to work with
Action Pack. This module only intends to provide guidance in case you want all
features out of the box.